1. Field of the Invention
This invention relates to a process for coating a substrate with alkaline or alkaline earth metals and more particularly lithium.
2. Description of the Prior Art
Presently, there is a high level of interest in industry in designing thin layer lithium batteries. Such batteries typically include a lithium anode, a transition metal oxide-polymer composite as a cathode, and an electrolyte which may be a solid or a liquid and which includes a dissolved lithium salt.
A principal objective of the designers of these batteries, particlarly in applications in wihch large electrode areas are needed, is to make them as thin as possible while satisfying market needs in terms of capacitance, current density, shelf-life and the like.
While methods for making lithium anodes are known, these methods typically provide an anode containing much more lithium than is necessary to meet the electrochemical requirements of the cell. As a consequence, lithium is wasted, the battery is more expensive, and the battery is substantially thicker and heavier than necessary. For example, the most common method for fabricating lithium anodes is cold rolling of lithium metal onto a substrate, however it is difficult to roll lithium metal into strips thinner than about 50 microns. U.S. Pat. No. 3,721,113, describes a method for alleviating this difficulty by rolling the lithium between smooth polymeric surfaces having sufficiently low critical surface energy to prevent adhesion, however, this method is limited to thicknesses of not less than about 40 microns.
Alkaline and alkaline earth metals are highly reactive metals, such metals and particlarly lithium react violently with moisture, are easily contaminated and require special handling.
A further difficultly involved with producing lithium coated substrates for later incorporation into batteries is that the surface of the lithium metal is subject to contamination which must be removed prior to forming into a battery, requiring special handling and expense.
Other methods for coating a substrate with lithium are known in the art as illustrated by U.S. Pat. No. 3,551,184 to Dremann, et al., which involves rubbing a heated substrate with a rod of lithium metal, and U.S. Pat. No. 3,928,681 wherein metal substrates are coated as they are conveyed through an alkali metal melt. Each of these methods has drawbacks which would make it difficult to implement in an industrial setting and neither provides a solution to the problems that result when the coatings are much thicker than required in a lithium cell. Additionally such composite structures are subject to delamination.
Methods for spray coating metals are known in the art as illustrated by U.S. Pat. Nos. 3,845,901 and 4,311,275, but none of them solves the problems associated with the use of alkaline and alkaline earth metals and particularly with lithium.